1. Field of the Invention
The present invention relates to a sheet supplying apparatus used with an image forming apparatus such as a printer, a copying machine, a facsimile and the like.
2. Related Background Art
An example of a conventional sheet supplying apparatus will be explained with reference to FIGS. 22 to 25. In FIG. 22, a sheet supplying cassette 106 contains a plurality of sheets S therein, and a sheet stacking plate 105 on which the sheets S are stacked is mounted within the sheet supply cassette 106 for pivotal rotation with respect to the cassette around a support shaft 105a.
Separation pawls 101 serve to regulate both downstream upper end corners of the sheet stack S so that an uppermost sheet S can be separated from the other sheets when a sheet supplying operation is started. A pick-up roller 103 is rotated around its own central shaft 103a in a direction shown by the arrow p to pick up the sheet S from the sheet supply cassette 106. The picked-up sheet S is conveyed into the interior of a body of an image forming apparatus (not shown).
An elastic member 102 serves to afford sheet supply pressure R to the pick-up roller 103. The elastic member 102 is disposed below the sheet stacking plate 105 and normally biases the sheet stack S and the sheet stacking plate 105 toward the pick-up roller 103 with a biasing force F shown in FIG. 23. In general, the sheet supply pressure R is defined by a force obtained by subtracting a gravity force of (a weight) of the sheet stack S from the biasing force F of the elastic member 102. This is the same in the following explanation.
As shown in FIG. 23, the pick-up roller 103 is constituted by a cylindrical surface portion and a flat surface portion so that it has a semi-circular (D-cut) cross-section. In an inoperative condition, the flat surface portion is directed downwardly to face the sheet stack S so that the flat surface portion is not contacted with an upper surface of the sheet stack S a height of which is regulated by the separation pawls 101.
In response to a sheet supply signal, when the pick-up roller 103 is rotated in the direction p in FIG. 23, as shown in FIG. 24, a forward end portion 103b of the cylindrical surface portion of the pick-up roller 103 is firstly contacted with the sheet stack S. In this case, the sheet stack S and the sheet stacking plate 112 are urged upwardly in opposition to the sheet supply pressure R (FIG. 24), with the result that the uppermost sheet S is displaced to form a loop portion Sa as shown in FIG. 24.
When the pick-up roller 103 is further rotated in the direction p (FIG. 24), as shown in FIG. 25, a tip end of the uppermost sheet S overrides the separation pawls 101, with the result that the uppermost sheet S alone is sent to the pair of convey rollers. The pick-up roller 103 is further rotated in the direction p. When the pick-up roller is rotated by one revolution, a condition shown in FIG. 23 is restored, with the result that the pick-up roller does not act on the sheet during further conveyance of the picked-up sheet S and is prepared for a next sheet supplying operation.
FIG. 26 shows a conventional electrophotographic image forming apparatus having the above-mentioned sheet supplying apparatus. In this image forming apparatus, a sheet supply cassette 202 is removably mounted within a body 201 of the image forming apparatus (the cassette can be dismounted from the apparatus along a direction shown by the arrow A), and there are disposed a sheet supply roller 203 for supplying a sheet S, a feed roller 204 and a retard roller 205 which are disposed at a downstream side of the sheet supply roller and which serve to separate and convey the sheet S.
The retard roller 205 is urged against the feed roller 204 by a biasing means (not shown) from below with a predetermined force F, and the feed roller 204 is drivingly rotated in a direction (for conveying the sheet S in a downstream side) shown by the arrow c by a drive means (not shown) provided within the body 201 of the image forming apparatus. Further, the retard roller 205 is connected to a drive shaft of the drive means (not shown) provided within the apparatus body 201 via a torque limiter, so that the retard roller is rotated in a direction (for conveying the sheet S in an upstream side) shown by the arrow d under the control of a torque action of the torque limiter.
Since the retard roller 205 is urged against the feed roller 204, when a rotational driving force of the feed roller 204 (directing toward the direction c) acts on the retard roller 205 to apply predetermined torque acting toward a direction e (opposite to the direction d) to the retard roller 205, slip is generated in the torque limiter, with the result that the retard roller 205 is driven by the rotation of the feed roller 204 to rotate in the direction e. That is to say, although the retard roller 205 is rotated in the direction d to return the sheet S within the predetermined torque, when torque greater than the predetermined torque acts in the sheet conveying direction (direction e), the retard roller is rotated to convey the sheet S in the downstream direction. The torque at which the rotational direction of the retard roller is changed in this way is referred to as "return torque" (Ta) of the retard roller hereinafter. In this way, the sheets S are separated one by one between the feed roller 204 and the retard roller 205, and the separated sheet is conveyed in the downstream direction.
However, in the above-mentioned conventional technique, in case of the pick-up roller 103 shown in FIGS. 22 to 25, since sheets S having various sizes and/or weights are generally used, the sheet supply pressure R is not normally uniform or constant. That is to say, due to the gravity force acting on the sheets S, when light sheets S having small size are used, the biasing force F is increased to thereby increase the sheet supply pressure R; whereas, when heavy sheets S having large size are used, the biasing force F is decreased to thereby decrease the sheet supply pressure R.
Although the pick-up roller 103 serves to pick up the uppermost sheet S alone, if the sheet supply pressure R is too great, several sheets S including the uppermost sheet will be picked up by the pick-up roller (double-feed), whereas, if the sheet supply pressure R is too small, the pick-up roller 103 will be idly rotated on the sheet stack S not to supply any sheet. That is to say, both if the sheet supply pressure R is too great and is too small, the poor sheet supply will occur. In order to avoid this, it is necessary to maintain the sheet supply pressure R within a proper range.
However, as mentioned above, since various kinds of sheets are used, some sheets do not accommodate with the proper sheet supply pressure R set in the sheet supplying apparatus, thereby causing the poor sheet supply. Thus, it is necessary that an adjusting mechanism for increasing and decreasing the biasing force of the elastic member 102.